The invention is in the field of optics, and specifically relates to optical waveguides.
Roughness scattering is one of the major sources of transmission loss in dielectric optical waveguides. The roughness at the core/cladding interface, where an abrupt change in the refractive index occurs, is responsible for such a scattering. It is particularly severe for high index difference (difference in the refractive indices between the core and the cladding) waveguides, since the scattering loss scales with index difference. The relationship between the scattering loss and the index difference can be found in Journal of Lightwave Technology 12, 790-796 (1994) by Suzuki et. al.
A high index difference waveguide typically has an index difference equal to or larger than 0.3 in a square channel waveguide configuration. A channel waveguide is a dielectric waveguide whose core is surrounded by a cladding that is composed of a material or materials with refractive indices lower than that of the core, and wherein the peak optical intensity resides in the core. High index difference waveguides can be defined in other waveguide geometries. In waveguide configurations that are difference from a channel waveguide, a high index difference waveguide is defined as one that has a mode-field size similar to that of a high index difference channel waveguide (within 50% difference in cross-sectional area). In these waveguides, a cladding is defined as a region where the evanescent field of optical modes exists.
Graded index waveguides are conventionally used in fiber optics to minimize modal dispersion. U.S. Pat. No. 4,412,722, issued to Carnevale et al. shows optical fiber waveguide with its index of refraction graded in the radial direction so as to yield low total dispersion. The index gradient is achieved by a diffusion process.
Grading the index at the core/cladding interfaces in planar waveguides is also shown in the prior art. Many publications including Applied Optics, Vol. 25, No. 9, May 1986 by Danko et. al. and Proceedings of SPIE, Vol. 2943 by Augusciuk et. al., show the fabrication of graded index channel waveguides by an ion-exchange process. In an ion-exchange process, the index of the waveguide core is raised above the waveguide cladding by inserting ions into the targeted region and annealing the waveguide to form a new chemical compound with a high refractive index than the previous material.
Proceedings of SPIE, Vol. 1774 by Shih et. al. shows a graded index waveguide that is formed by photolithography. A photo-sensitive polymer is used as the waveguide material that changes the index according to the intensity of the light that impinges on it.
In the prior art, waveguides are formed by methods of increasing the index of the targeted region, which becomes the waveguide core, over the rest. As a result of these methods, the index is graded from the core to the cladding.
The graded index waveguide of the invention reduces the effect of the core/cladding interface roughness on the scattering loss, therefore reducing the loss. By gradually changing the index at the core/cladding interface, the scattering loss is minimized, since the index jump from the core to the cladding is moderated by the presence of the graded index layer at the interface. The gradual index change at the interface results in the lower scattering loss since the mode traveling in the waveguide experiences less index difference at the core/cladding interfaces.
In accordance with one embodiment, the invention is a planar waveguide that has a graded index layer between the core and the cladding of a dielectric waveguide, created by either deposition, growth, or a chemical reaction, before the final cladding is put down. The graded index layer is created after the waveguide core is already formed. This is different from the prior arts where the graded index profile is automatically created when the waveguide core is being formed. In this invention, a graded index profile exists at the core/cladding interface, reducing the scattering losses due to interface roughness.
It is an object of the invention to minimize scattering loss by altering the abrupt index jump from that of the core to that of the cladding of a planar waveguide, by grading the index at the interface. Many index profiles at the waveguide core/cladding are possible for reduction of the waveguide loss. It is another object of the invention to show that such a graded index interface between the core and the cladding can be achieved by methods of adding materials such as thin film growth or deposition on the waveguide core. Such a graded index interface between the core and the cladding can be achieved by methods of altering materials such as a chemical reaction on the surface of the waveguide core. It is yet another object of the invention to minimize modal and polarization dispersion of a planar waveguide using the graded index design.